As a thought, not tested, using wifi probe requests might be better through more dense material scenarios. Using a specific AP probe request "SOS Request" from the device would help discover over all the other AP requests that others are throwing. No idea if iOS or Android would let you hijack that process.

This is a bit more extreme, but some type of triangulation from multiple rescuers could be useful in closing in on a spot.

This is really interesting and thanks for sharing.